Effluent water containing metal ions such as chromium, lead and zinc ions, is discharged in many branches of industry. In the galvanizing industry, washing after various chemical and electrochemical processes for the surface treatment of metals is the most important cause of the presence of heavy metal ions, for example chromium, silver, cadmium, iron, copper or nickel ions, in effluent water. Effluent water containing chromium compounds is discharged by tanneries. Mercury ions are found in effluent discharges from mercury-electrolysis factories. In the organic process industry, where for example oxidations with chromic acid or lead acetate and reductions with zinc are performed, effluent water containing chromium, lead and/or zinc ions is discharged. This effluent water from the organic process industry contains a relatively large amount of metal ions, for example chromium from 1 to 5% by weight and lead to even 10% by weight.
In connection with environmental requirements, the effluent containing heavy metal ions may not be discharged untreated into the sewer or into public waters, so that the indicated route is removal of the metal ions before discharge. Precipitation in the form of metal hydroxides or basic metal carbonates, brought about by the addition of basic substances such as NaOH, CaO, Ca(OH)).sub.2, Na.sub.2 CO.sub.3 and MgO, is therefore a self-evident method.
In practice, the precipitation method is not free from its problems. The precipitates formed, in particular the metal hydroxide, are generally gelatinous and extremely bulky, and because of their colloidal nature they are difficult to remove by filtration. Slurries with a dry matter content of 1% may still be pumped, but when an aqueous slurry has a dry matter content of 5 to 15% it may scarcely be pumped. Filtration of the slurry is also difficult and slow. With the filters which are in general use in the technique, filter-cakes with a dry matter content from approximately 10% to about 20% are obtained.